1. Field of the Invention
The present invention relates to the heating of fluids and particularly to the heating of air for injection into a shaft furnace. More specifically, this invention is directed to heat exchangers and especially devices known in the art as hot blast stoves which are employed to heat gases to high temperatures. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use with equipment which is ancillary to a blast furnace. Present blast furnace installations are provided, exterior of and adjacent the lower portion of the furnace proper, with an annular conduit which is known as the "hot blast conduit." The "hot blast conduit" is connected to the interior of the blast furnace via a number of transmission conduits known in the art as "tuyere stocks." Preheated air, known as "hot blast," is introduced into the blast furnace via the "hot blast conduit " and "tuyere stocks." The heating of the air to be injected into the furnace is performed in an ancillary device known as a "hot blast stove." Means, typically in the form of a mixing chamber, are interposed between the hot blast stove and the hot blast conduit to mix the heated air with cold air in the interest of regulating the temperature of the air which is introduced into the furnace to this maintain the injected air temperature constant. In a modern blast furnace the controlled temperature of the injected hot air may reach 1350.degree. C.
A hot blasst stove is functionally divided into two sections. The first section, which is a combustion chamber, is provided in its lower part with a burner. A combustible gas; typically a blast furnace gas enriched with coke gas, natural gas, or the like; is delivered to the burner. The combustion products; i.e., the thermal energy and heated gas resulting from burning the combustible gas in the presence of air; are directed through a second section of the hot blast stove, known as the checkerwork chamber, which is comprised of refractory bricks. Heat from the gases passing through the checkerwork chamber is transferred to and stored in the checkerwork of refractory bricks.
The operation of heating air in a hot blast stove is essentially a two step process. In the first step or period, known as the "stove on gas" phase, the combustible gas is burnt in the combustion chamber and the resulting combustion products ascend in the combustion chamber and then descend through the refractory checkerwork before delivered to an exhaust stack. The heat emitted by the combustion products is, as noted above, stored in the refractory material of the checkerwork.
During the second step or portion of the air heatng cycle, termed the "stove on blast" phase, "cold" air is introduced at the base of the refractory checkerwork at a pressure which may be in the range of 5-7 atmospheres. During the "stove on blast" phase the gas circulation in the hot blast stove is in the opposite direction of that which occurs during the "stove on gas" phase; i.e., the air passes through the refractory checkerwork first and then passes to the hot blast conduit via the combustion chamber. While passing through the checkerwork the air recovers heat which was stored in the checkerwork during the "stove on gas" phase.
As will be obvious from the above discussion, since a hot blast stove has two separate modes or phases of operation, a blast furnace installation requires the presence of at least two hot blast stoves to satisfy a continuous demand for preheated air; i.e., for a "hot blast." In a two stove installation one hot blast stove will be in the "stove on gas" phase while the other will be in the "on blast" phase.
In conventional hot blast stoves the combustion chamber and the refractory checkerwork are incorporated in a single brickwork structure which is a few dozen meters in height. In such hot blast stoves, known in the art at "hot blast stoves with incorporated combustion chamber," the combustion chamber is adjacent to the checkerwork and separated therefrom by a wall of refractory material. Prior art hot blast stoves with an incorporated combustion chamber suffer from the drawback of undergoing rapid deterioration. This deterioration is caused by gaseous short circuits which occur between the combustion chamber and the checkerwork. These short circuits lead to the destruction of the refractory material as a consequence of heat surges resulting from the considerable differences between the temperature prevailing on the opposite sides of the dividing wall. The increasingly high temperatures required for the hot air which is injected into modern blast furnaces have the effect of accelerating this deterioration.
Hot blast stoves wherein the combustion chamber is separated from the refractory checkerwork have been designed in an effort to enhance the operating life of the stoves. These devices, known as "hot blast stoves with separate combustion chamber," comprise two separate chambers; i.e., a combustion chamber and a checkerwork chamber; which are in communication via a cupola. For the reasons to be set forth below, the use of separate chambers has not solved the problem of hot blast stove deterioration in the face of the increasingly high temperature requirements for the hot blast air to be injected into a modern blast furnace. In hot blast stoves with separate combustion chambers the air may reach a temperature of 1500.degree.-1550.degree. C in the zone of the cupola. This enables a controlled temperature of 1350.degree. C to be obtained for the air to be injected into the furnace.
Attempts have been made to prevent the deterioration of hot blast stoves by resort to new refractory materials; particularly materials with a silica base. Additionally, steel jackets have been installed in the cupolas of hot blast stoves which operate at very high temperatures and high pressures. For the reasons to be set forth below, the use of the most advanced refractory materials and the employment of steel jackets have not solved the deterioration problem.
An unexpected problem, which has been termed "intercrystalline stress corrosion," has recently manifested itself. This "intercrystalline stress corrosion" causes deterioration of the steel jackets of the cupolas of hot blast stoves operating at high temperatures and pressures. The intercrystalline stress corrosion phenomenon is due to the simultaneous existence of three conditions; i.e., high temperature, high pressure and the existence of ions of nitrous oxide, chlorine and sulfur.
The nitrous oxide ions form during the combustion of gases at a high flame temperature; i.e., above 1300.degree. C; in the combustion chamber, during the heating of the "cold" air to temperatures in excess of 1400.degree. C and during contact of the air with the bricks of the hot blast stove which are themselves heated to the range of 1500.degree.-1550.degree. C. The chlorine and sulfide ions are introduced by the insufficiently purified combustible gas delivered to the burner in the combustion chamber.
The steel plates or jackets of the cupola of a hot blast stove are subjected to comparatively large physical stresses as a result of both the residual stresses produced by the welding steps during fabrication and from the pressure exerted on the hot blast stove during the "stove on blast" operational mode. The repeated application and removal of pressure during the operation of a hot blast stove will ultimately lead to the creation of microcrystalline cracks in the steel jacket. These microcrystalline cracks present no problem in themselves. However, the condensation of nitrous oxide, chlorine and sulfide ions in these cracks results in occurrence of the intercrystalline stress corrosion phenomena.
To summarize, in order for the intercrystalline stress corrosion phenomena to occur within the cupola of a hot blast stove the above-mentioned three conditions must simultaneously occur; i.e., there must be a high temperature, a high pressure and the condensation of nitrous oxide, chlorine and sulfide ions. The microcrystalline cracks in the steel blast stove cupola jacket can not form without the existence of high pressure and pressure fluctuations. The ions of nitrous oxide, chlorine and sulfur must be formed, requiring a high temperature, and must condense in the cracks. As will be obvious to those skilled in the art, the problem of intercrystalline stress corrosion can not be overcome by reducing the pressure within the hot blast stove or by reduction in operating termperature; both of these potential solutions being at variance with modern blast furnace technology which requires high "hot blast" air pressures and temperatures for increased productivity and enchanced quality.
In order to alleviate the intercrystalline stress corrosion phenomena it has been suggested that the internal wall of the cupola of a hot blast stove be provided with a coating of aluminum in the form of either an aluminum base paint or actual sheets of aluminum. This proposed solution, however, has not proven successful since the high temperatures and pressures in the cupola zone of hot blast stoves results in failure of such coatings.
To briefly summarize, despite all efforts made to date, an effective means and apparatus for preventing intercrystalline stress corrosion to occur in and thus limit the operational life of a cupola of a hot blast stove has not previously been devised. As a result of this failure to provide an effective means of preventing intercrystalline stress corrosion, and because of the frequent maintenance which must accordingly be performed on the hot blast stoves, progress toward use of higher hot air temperatures and pressures in blast furnaces has been abortive.